This application relates to image processing in digital cameras and other electronic digital image acquisition devices, and particularly to the correction of chromatic aberration and the calibration of the device for this purpose.
For the optical material used in lenses, light of different wavelengths will have different indices of refraction. As a result, lenses typically have some degree of lateral chromatic aberration. Lateral chromatic aberration appears as false colors in an image that grow stronger as the distance from the center of the image increases. Improving the optics can reduce the amount of aberration, but digital technology allows for the ability to “fix” the image by using one color channel (such as green in a RGB breakdown) as a reference channel and compensating the other channels (such as red and blue) with respect to the reference channel.
Typically, digital cameras use chroma low-pass filtering to remove false colors in the image, including those created by lateral chromatic aberration. However, this solution has several shortcomings and will generally not provide sufficient image quality for several reasons. A first of these is that in the case of a strong chromatic aberration, chroma low-pass filtering will not be able to remove the false colors effectively, but will instead smear them. Another is that chroma low-pass filtering will blur the chromatic edges in the image. Of course, another way to address the problem is by simply enhancing the optical system of the camera; however, this may be prohibitively expensive for most applications. A third method employed by prior art systems is to correct lateral chromatic aberration outside the camera, according to lens data and focal length information, acquired when the image was captured and saved together with the image.
As the trend is to lower lens cost, because the lens is a significant portion of the camera's bill of material, it is likely that lower cost lenses will be increasingly used. This will cause chromatic aberration effects to become even a greater problem. Consequently, the need for improved digital image processing solutions to resolve chromatic aberration effects is increasing, since they facilitate digital camera cost reduction, without a concomitant decrease in image quality. This is particularly true of methods that can be executed on camera without increasing image processing time, as camera manufacturer's demands in terms of reduced click to click time, are increasing. Further, since as aberration is lens dependent, there is also a needed for improved methods for calibrating the aberration correction mechanism for the lens being used. The need for improved calibration is present not just for camera bodies that can accept different lens, but also for camera with a single permanently attached lens, as these tend to have less expensive optics where the actual Lens values are more likely to differ from the expected nominal values.